fix x2 without gNB
This merge request contains two things:
- fix x2 without gnb (that is: the eNB is running without gNB, then we must not switch the UE to NR)
- exit gNB if eNB crashes (and exit eNB if gNB crashes)
I'm okay if the second point is not accepted. Note also that we can accept it and change the behavior later on (that would be: cleanup the state of the component that keeps running, accept to recreate the connection when the other component is restarted).
See the commit messages for more details.